This invention pertains to drilling the curved build section of a drainhole from a generally vertical section of a wellbore. More paticularly, this invention relates to a drainhole drilling assembly with an elliptic drill collar oriented in the vertical plane to produce significant torsional resistance to rotation.
In the art of producing hydrocarbonaceous substances such as crude oil from subsurface formations, drainholes are used in various ways to increase production. Drainholes are wellbores that deviate from a generally vertical or a generally vertically oriented slanted wellbore toward a more generally horizontal direction. Drainholes have a curved or build section in which the borehole is turned from the vertical direction toward the horizontal direction until the desired more horizontal direction is reached. Thereafter, a straighter more stabilized section is drilled, for example, a section of much larger radius of curvature. Drainhole wellbores may include more than one curved and more than one stablilized section deviating from the same vertical wellbore. Drainholes may also include deviations from the stabilized section and one or more curved build intervals. Drainholes may also include dropping sections or intervals. This disclosure pertains primarily to a system for drilling the build section of a drainhole.
Various assemblies or systems have been proposed for developing and drilling the curved build section of a drainhole. For example, U.S. Pat. No. 3,398,804 describes a system comprised of knuckle joint sections, a reamer and a drill bit. The initial deflection and direction of the curved build interval is controlled by a deflecting tool or whipstock which in combination with the knuckle joints and forces on the drilling assemblege causes the drill bit to cut into the wall of the borehole, thereby tilting the common bit-reamer axis from the axis of the wellbore. As drilling is continued, the wellbore curves in the desired direction. In this patent, torque is applied to the reamer and bit by rotation of the drill string. In this invention, a conventional downhole motor turns the bit.
The wellbore curvature is commonly stated in terms of degrees per foot. U.S. Pat. No. 3,398,804 and conventional downhole motor drainhole drilling systems refer to large angles of build. For example, U.S. Pat. No. 3,398,804 addresses angles from 0.2.degree. per foot to 20.degree. per foot with 10.degree. to 12.degree. per foot being preferred. This is a radius of about 4 to 6 feet. This invention refers to medium angles of build of 0.5.degree. or less per foot with angles less than 0.2.degree. per foot being preferred.
Other types of drainhole drilling systems also use downhole motors to turn the bit and a bent sub or a bent housing to generate the desired wellbore curvature. Downhole motors and their housings vary in length, but the overall length rarely exceeds 34 feet. Prior art systems using a downhole motor to drill a drainhole refer to relatively shallow wells with only a very few thousand feet of drill pipe involved. These systems concern build intervals with large angles of build. The curved section of the wellbore is near the top of the hole. Downhole motors produce torque forces on the drill string. When the curved interval of the drainhole is near the top of the hole, it is not too difficult to select an orientation from the top of the drill string that provides for the required torsional rotation of the drill string due to the torque produced while drilling with a downhole motor. The accuracy of this selection governs whether the build assembly builds in the intended direction or also produces a curve in the horizontal plane either to the right or left of the intended target. With only a very few thousand feet of drill pipe involved, conventional prior art downhole motor systems produce good results even though the torque produced by the downhole motors or turbine is quite variable. But these systems are unreliable for deeper wells.
This invention pertains to drilling full size drainholes with medium radius of curvature build intervals at much greater depths than previously used. The torque on the drill string and subs or housing and the orientation of the bit is much more difficult to control and maintain. For example, the resistance of axial angular deflection of 7000 feet of 2 7/8 inch drill pipe held at the surface is only 0.8 ft-lbf per degree of curvature.